The present application relates to epitaxial growth, and especially to epitaxial growth of monocrystalline semiconductor layers on monocrystalline substrates composed of a different material (“heteroepitaxial deposition”).
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
One of the basic steps in semiconductor processing is deposition of thin layers of material. Epitaxy is the special case where crystalline material is deposited onto a monocrystalline substrate, under conditions where the new material continues the same crystal lattice, with the same orientation, as the substrate.
The specific case where the added material has a different composition from the substrate is sometimes referred to as heteroepitaxial deposition. Heteroepitaxial deposition is easiest when the lattice constant of the epitaxial material is equal to that of the substrate material: otherwise there will be strain (tension or compression) in the material. However, perfect lattice match is often impossible.
Molecular Beam Epitaxy (“MBE”) is one example of an epitaxial deposition process. In this process a substrate is held under very high vacuum, and usually heated (e.g. to a few hundred degrees Celsius). Source atoms or molecules are released very slowly to impinge on the exposed substrate, to permit slow crystal growth.
Epitaxial deposition can also be performed from the vapor phase. Such processes use a variety of source gasses, and typically achieve much faster crystal growth. Epitaxial growth from liquid or solid phases is also possible.
A particular challenge is preparation of the substrate before epitaxial deposition. The epitaxial deposition process requires that the incoming atoms (the “adatoms”) be preferentially attracted to the lattice sites, in the exposed surface of the crystalline substrate, which will continue the substrate's lattice as crystal growth occurs. The problem is that any deviation from perfect alignment of the adatoms can initiate twinning or defects, which degrade the properties of the epitaxial layer. Once defects or twinned domains are initiated, they may propagate as the growth plane moves.
The alignment of each layer of grown crystal is defined by the crystal lattice layer just before it. This is what allows growth of a crystalline material, but this also permits defects in the existing surface to propagate into the new material as the crystal grows. In other words, each newly-deposited layer is defined by the surface layer, and not the bulk crystal, of the crystalline material it grows onto.
This is not a great difficulty during conventional growth processes, but it indicates one source of weakness: growth must be initiated at some point, and a good crystal lattice surface must be available, at that point, for the newly grown layers to align themselves. Much effort has been invested in developing epitaxial growth processes, but (depending on the substrate material) initiation of epitaxial deposition can still be an important source of defects.
Conventionally a CMP (chemo-mechanical polishing) step is used to provide an atomically ordered crystal lattice surface.
The present application relates to the preparation of solid crystalline substrate surfaces in conjunction with epitaxial growth of semiconductor layers upon that substrate. The quality of epitaxial layers (Epi layers) grown on substrates (e.g. InGaAs grown on GaSb substrates) depends upon the perfection of the atomic lattice at the surface of the substrate, and lack of interfering species on that surface such as oxygen, carbon, hydrocarbons, H2O, OH, and other species, which would disrupt the uniform growth of Epi layer atomic structure as an extension of the substrate atomic lattice.
Standard industrial practices in preparing a substrate for Epi growth involve a number of typical steps:
1. Chemical-mechanical polishing of the substrate to produce a pristine, ordered crystalline surface, free of lattice anomalies;
2. Cleaning of the substrate surface with organic solvents and acids after polishing to remove any residue from the polishing process;
3. Growth of an intentional oxide on the prepared surface, typically in a furnace with high-purity oxygen present;
4. Storing the oxidized substrate wafer for later use in a sealed container;
5. (Optionally) cleaning the surface of the substrate to remove organic contamination which has accumulated on the surface due to outgassing of storage packaging;
6. Placing the wafer into an epitaxial growth machine with high-vacuum capability;
7. Heating the wafer to very high temperatures in high vacuum in order to desorb organic contamination and also the grown oxide layer (from step 3);
8. Growth of the epitaxial layer(s).
Step 1 above uses lapping slurries, lapping pads, acid solutions and polishing pads to progressively remove sawing damage from the wafer and eventually produce a smooth surface with high crystalline regularity in the atomic lattice on the immediate surface of the substrate.
Step 2 above must remove all traces of lapping compounds and polishing chemicals from the surface of the substrate without disrupting or damaging the pristine crystalline lattice on the surface of the substrate. This is very difficult to do, and typically utilizes proprietary solvent and acid rinses which consume large amounts of these solvents and acids which must then be disposed according to increasingly strict environmental regulations. Extreme care is taken to remove as much of the lapping and polishing residues as possible without causing any disruption of the pristine surface crystallinity. Since the surface must not be contacted by any mechanical apparatus, complete removal of all residue is extremely difficult. Some substrate manufacturers have tried vacuum plasma cleaning to remove final residues, but the atomic bombardment in this type of plasma system damages the surface lattice structure, thus defeating the whole surface preparation process. Additionally, any process that requires the substrate to pass into and out of a vacuum chamber slows the throughput of the cleaning process. What is needed is a method of removing all polishing residue quickly, without contacting the substrate surface, without requiring a slow vacuum process, and without bombarding or damaging the surface in any way.
Step 3 above brings about the growth of an oxide protection layer on the surface of the substrate, which performs a number of functions:
a) It consumes some of the surface lattice atoms which might still contain some damage or crystalline irregularity from the cleaning process (step 2, above).
b) It creates a new pristine semiconductor surface at the base of the oxide layer.
c) The oxide prevents the pristine crystalline surface from gettering oxygen, carbon, hydrocarbons, H2O, OH, etc. during storage and/or transport of the substrate.
d) The oxide must be of a composition that is completely desorbed upon heating in high-vacuum Epi deposition equipment.
This oxide surface protection scheme has a number of drawbacks and challenges:
a) The oxide layer grown has a different lattice constant than the underlying semiconductor lattice, thereby inducing stress into the semiconductor right at the critical interface. This can result in dislocations, slip, and other crystalline lattice disruptions which are detrimental to subsequent Epi growth.
b) Typical Epi substrates are compound semiconductors, for example, GaSb. The oxidation rate of the cations (gallium, in this example) is typically different than the oxidation rate of the Anions (antimony, in this example), and so the oxide grown is often non-stoichiometric, imperfect, and strained. This results in stress, possible gettering of unwanted species such as carbon; and uneven desorption during the pre-deposition high-temperature removal of the oxide.
c) In some compound semiconductor schemes, certain oxidation states of anions or cations require extremely high desorption temperatures (undesirable).
As described below, the present application provides a method to protect the pristine crystalline surface, that does not damage the surface lattice, does not strain the surface lattice, that is uniformly effective at passivating the dangling bonds of the surface lattice such that they will not getter undesirable species, and desorbs uniformly and completely under modest heating in the Epi deposition system, thus leaving a pristine crystalline surface upon which to grow epitaxial layers.
Steps 4 and 5 above, are required if the substrates are produced in one location and used in another (which is most often the case.) Epi substrate wafers are typically stored and transported in individual polymer containers. These containers are known to outgas organic components onto the wafer surface. Some Epi growth labs assume these organics will desorb as the wafer is heated up in the high-vacuum deposition apparatus. However, other organizations realize that these organics will create problems in their high-vacuum pumping systems and decrease the required mean-time-between-system-cleans. These labs typically use solvent rinses of the substrate in order to remove these adventitious organic residues. However, the process of wet solvent cleans in less-than-perfect environments can introduce other contaminants to the substrate surface, which eventually end up in the high-vacuum deposition apparatus. Additionally, these solvents must be disposed of in environmentally sound ways, thus incurring significant additional cost. What is needed is a way to remove all organic contamination from Epi substrates before they go into the Epi growth system without introducing new contaminants, without damaging the surface, and without the use of costly and environmentally undesirable solvents.
Step 7 above (the desorption of the oxide protective film inside the Epi deposition apparatus) is typically the most critical step in conventional Epi substrate surface preparation. All of the oxide must be uniformly desorbed from the substrate surface in order to leave exposed the pristine crystalline substrate surface upon which the Epi films will grow. If there is any residual contamination from the polishing process, any residual organic contamination from atmospheric exposure or packaging outgassing, any non-stoichiometric oxides, or any high-temperature oxides, the Epi films grown upon the substrate will contain defects which can eventually cause device defects and yield loss in the subsequent semiconductor manufacturing process. What is needed is a method to protect the pristine crystalline surface, that does not damage the surface lattice, does not strain the surface lattice, that is uniformly effective at passivating the dangling bonds of the surface lattice such that they will not getter undesirable species, and desorbs uniformly and completely under modest heating in the Epi deposition system, thus leaving a pristine crystalline surface upon which to grow epitaxial layers.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.